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 Apr 5 comment Why is CRC said to be linear? Ah, that corrects a long-standing terminology problem I have had, with (wrongly) using linear where affine was meant in a cryptanalytic context! I'll have to scrub my earlier answers.. Apr 5 revised How to use the Extended Euclidean algorithm to invert a finite field element? typo Apr 5 revised How to use the Extended Euclidean algorithm to invert a finite field element? Expand Apr 5 revised How to reverse engineer a cryptography algorithm if some input-output pair is known. Polish Apr 4 answered How SAM modules secure transactions? Apr 4 answered How to reverse engineer a cryptography algorithm if some input-output pair is known. Apr 4 revised Purpose of DES parity bits Link to useful comment Apr 4 comment For RSA keys, is the safety of a given key-length different for signing vs encryption? I thing that you mean the other way around. It is always possible to re-sign the original message with a stronger key, and that gives the signature the strongest strength; but re-enciphering the original secret message with a stronger key is pointless if the adversary has the cryptogram made with the weaker one. Apr 4 comment For RSA keys, is the safety of a given key-length different for signing vs encryption? There is a reason sometime given to use longer/stronger keys for encryption of things that are intended to remain secret "forever"; when often, signatures become pointless after a shorter time. Apr 4 revised How to use the Extended Euclidean algorithm to invert a finite field element? fix a typo Apr 4 answered How to use the Extended Euclidean algorithm to invert a finite field element? Apr 4 revised 128 bit 3DES Key and AES Key: what's the difference? Move discussion about DES parity bits to another answer Apr 4 revised Purpose of DES parity bits Union of two versions Apr 4 revised Purpose of DES parity bits Picture of the evidence Apr 4 revised Purpose of DES parity bits Tag a history Apr 4 answered Purpose of DES parity bits Apr 1 awarded Popular Question Mar 31 comment Speed of a 16384-bit RSA key Also: it you stick to RSA with two prime factors of the public modulus, each will exceed the typical capability of hardware accelerators and tested-secure implementations, thus the very feasibility in secure conditions may be lost. A reasonable compromise could be using multi-prime RSA with 4 factors of 4096 bit each. If that's supported by the hardware (and a lot of lesser ifs) that should allow 16384-bit RSA private key operations to run only about 16 times slower than 4096-bit RSA private key operations with CRT and two factors, beating the $\Theta(b^3)$ scaling by a factor of about 4. Mar 31 comment In case M is small is it possible to recover the message That nice answer works even without using that the given $e=17$ is small. Mar 30 comment How to invert a linear hash function? Hint: write down (perhaps in the question) your definition of a linear hash function. Obtain $H(M_i)$ for all $M_i$ of some fixed size with exactly one bit set, and if that's not implied by your definition, the hash of the all-zero message with that fixed size. Show how you can constructively and efficiently compute a message $M$ of that fixed size with $H(M)=H(M′)$, given $H(M′)$, for any unknown $M′$ of that fixed size (we do not care if $M=M′$ or not). That achieves your goal.